WASHINGTON – By questioning the fundamentals of America’s global role, Donald Trump has given Hillary Clinton a chance to lift her game — by explaining why continued international engagement is in America’s interest and the world’s.

If Clinton can’t counter Trump’s “America first” rhetoric, and make the case that U.S. leadership is still crucial for our security, she won’t be a strong president. And she won’t have public support for the policies needed to rebuild American credibility.

Trump’s critics sometimes argue that his neo-isolationist views are so extreme and dangerous that they shouldn’t have a platform. But the polls make clear that many Americans agree with him. The questions he’s raising about unsuccessful foreign wars and ungrateful allies deserve good answers.

A real debate about America’s role abroad would be one of the benefits of this campaign, lifting it, at least partially, out of the rancorous mudslinging that’s ahead. Trump caricatures America’s alliances in Europe and Asia; Clinton needs to explain why they matter.

We won’t know officially until July if Trump and Clinton will be the nominees, but it certainly looks that way now. This matchup gets sneers from many commentators, but it would be a good test for both. Mr. Reality Television will meet reality. The author of “Hard Choices” will have to show she can make them, and explain them to the country.

The great election campaigns in American history — think 1860, 1932, 1940 — came at inflection points for the country. The nation faced existential crises, and the public was sharply divided. The elections clarified choices and made good leaders better.

Abraham Lincoln won a four-way race in 1860 on an anti-slavery platform that helped trigger Secession; Franklin Roosevelt won in 1932 with a mandate for what became the New Deal; his triumph in 1940 over Wendell Willkie was a repudiation of the isolationist movement and moved the nation toward eventual war with Germany and Japan.

The election of 2016 is a similar fork in the road. Trump, the insurgent candidate, is challenging many of the foreign policy assumptions that have prevailed since 1945. He thinks NATO is too expensive and that Japan and South Korea should defend themselves, with nuclear weapons, if necessary.

Trump’s venomous rhetoric about Mexicans and Muslims obscures the core of his message — and its appeal for many Americans who are tired of paying the bills for others’ security. Many commentators savaged Trump’s April 27 foreign policy speech, in effect likening it to putting lipstick on a pig, but he made five serious arguments: America’s resources are overextended; U.S. allies aren’t paying their fair share for defense; America’s friends think they can’t depend on us; rivals no longer respect us; and America lacks clear foreign policy goals.

Take the question of NATO: It’s an unfortunate fact that support for the trans-Atlantic alliance is gradually eroding in the United States. A poll last year by the Pew Research Center showed that just 49 percent of Americans had a favorable view of NATO, compared with 53 percent in 2009. That’s why Trump’s diatribes resonate with his neo-isolationist base.

Clinton needs to answer this critique directly. She needs to explain why America has a stake in a messy world. She needs to offer a clear explanation of how she would restore the credibility of American power without entangling the country in unwinnable new conflicts. She’s seen as more willing to use military force than President Barack Obama, but what would that mean in Syria, Iraq, Ukraine and the South China Sea?

One reason Clinton should welcome the bruising campaign ahead is that if she can rebut Trump, she has a better chance to be the strong president that America and the world need. A Pew poll back in January found that just 35 percent of the public thought Clinton would be a “good” or “great” president. Trump’s number was worse, but that shouldn’t be reassuring. Clinton doesn’t just need to win; she needs to learn to be great.

Bob Gates, the former defense secretary, put the matter bluntly in accepting an Atlantic Council award here Tuesday. “Contrary to the views of some politicians, continuing American global leadership is in our own economic, political, and security interest. … America turning inward not only will make the world more dangerous for others, but also for us.”

Clinton’s challenge is to convey this message in a way that connects with the voters. If she can do that, she’ll probably win — but, more important, she’ll be better able to lead the country as president.